Many electronic devices and electrical systems, such as transistors, integrated circuits, power controls, switches, microprocessors, and the like, generate heat during operation. The capability of some electronic devices is limited by their ability to remove or expel internally generated heat. This heat should be removed from the device to avoid general or localized thermal degradation or failure of the device. In some devices, the heat generated is sufficiently dissipated to the surrounding environment by the enclosure, package, header, or leads. Other devices require additional heat dissipating apparatus, such as heat sinks, heat exchangers, etc., for removing and dissipating excess thermal energy. For purposes of the present invention, a heat sink is any body of thermally conductive material such as metal or other like material which is placed in thermal communication with an electronic device package or other heat generating component for transferring internally generated heat from the device and rapidly dissipating this heat to the surrounding environment by conduction, convection, and/or radiation. Heat sinks may be extruded, machined, molded, sawed, or formed of sheet metal bodies.
It is instructive to consider the dissipation of heat from a heat generating device in terms of heat transfer, that is, the movement or transfer of heat from the device to the heat dissipating apparatus where the heat may be efficiently dissipated. The term "thermal path" will be used herein to refer to the path along which the heat is transferred from the heat generating device through the heat dissipating device (generally a heat sink) to the surrounding environment. A typical thermal path for a heat dissipating assembly would be as follows: the heat is generated by a heat generating electronic device package; the heat travels from the device package through a first thermal interface between the device package and a heat sink; the heat travels through the heat sink; the heat travels through a second thermal interface between the heat sink and the surrounding environment; and the heat is then dissipated into the surrounding environment. In order to ensure that the heat can be dissipated from the heat generating device at a sufficient rate, the heat must be able to travel from the heat generating device to the dissipating environment at a rate commensurate with the rate at which the heat is being generated. Accordingly, the heat must be able to travel along the thermal path as efficiently as possible. Thus, each step in the thermal path from the heat generating device to the dissipating environment must be designed to maximize the efficient transfer of heat. For example, to maximize the rate at which heat can be transferred through the heat sink itself, heat sinks are generally made of materials having high coefficients of thermal conduction such as aluminum, copper, and alloys thereof. Similarly, since a typical heat sink for electrical applications functions by conducting heat away from the heat generating component and dissipating the heat into the surrounding air, heat sinks are typically shaped to maximize surface area by incorporating fins or pins. Increasing the heat sink's surface area increases the physical size of the thermal interface between the heat sink and the surrounding atmosphere (the second thermal interface referenced above), thereby increasing the heat sink's ability to dissipate heat to the surrounding atmosphere.
Of particular interest to the invention at hand, is the first thermal interface, i.e., the thermal interface between the heat sink and the heat generating device package. In order for the heat generated to efficiently travel from the heat generating device to the heat sink, the heat sink must be placed in efficient thermal communication with the heat generating device package. Generally, the most efficient thermal communication can be achieved by securing the heat sink directly to the heat generating device package. Various means have been used to attach heat sinks in efficient thermal communication with heat generating device packages. A known practice is to glue, solder, or otherwise adhere a heat sink directly to a heat dissipating surface of the body of a heat generating device package with heat-conductive epoxy, solder paste, thermally enhanced adhesives, or the like. Heat sinks may also be mechanically attached to heat generating device packages with resilient metal clips mounted on the heat sink or with screws, bolts, clamps, or other connective means which urge the heat sink and electronic device package into mutual physical contact. Although typically not as efficient, heat sinks may also be remotely located but thermally coupled to a heat generating device via a heat spreader device, a heat pipe, or any other means of transferring heat from the source of the heat to the heat sink.
Recently, technological advances have allowed electronic components to decrease in size while significantly increasing in power and speed. This miniaturization of electronic components with increased capability has resulted in the generation of more heat in less space. As a result, the electronic device packages have less physical structure for dissipating heat and less surface area for attaching a heat sink to dissipate the heat. The reduction of surface area available to attach a heat sink or other heat dissipating device reduces the effective thermal path for the heat to move from the heat generating device to the heat dissipating device. A smaller thermal path means less heat can move from the heat generating device to the heat sink; thus, the heat is dissipated at a slower rate and ultimately less heat can be dissipated.
Further complicating these general thermal management issues is the growing preference for surface mounting electronic components on printed circuit boards (PCBs) or other substrates. The use of surface mount PCBs or substrates has become increasingly popular because such substrates allow for a less costly and less time consuming process of fabricating and populating the PCB. As opposed to the manufacturing assembly process of older substrates which required insertion of components through holes in the circuit board for subsequent soldering operations, surface mount PCBs allow for the increased use of automated manufacturing and assembly techniques. In particular, surface mountable devices are typically robotically picked and placed on the PCB and then soldered to the PCB in one automated manufacturing process. In addition to reducing assembly costs, however, the surface mount technology has also allowed for even greater miniaturization of the electronic device packages used on the boards. These smaller surface mount device packages further reduce the device's ability to dissipate its own heat, thus increasing the need for separate heat sinks. In addition, the smaller packages make it increasingly difficult to attach a heat sink directly to the device package. Finally, even when a heat sink can be attached directly to the heat generating device package, the efficiency of the thermal path is limited by the available contacting surface area on the smaller device package.
Several methods have been suggested to effectively dissipate heat from these smaller surface mount electronic device packages. One common approach is to use the ground plane, or other similar thermally conductive area of the PCB (such as a thermal plane, thermal pad, or thermal land) as a rudimentary heat sink to spread and dissipate the heat directly from the PCB. If the ground plane is used as a thermal plane, heat from the electronic device package can be transferred to the thermal/ground plane via the ground leads of the electronic device package. Additionally, if the electronic device package has a collector tab, or other heat dissipating tab, this tab can be thermally coupled to the thermal plane of the PCB via a thermal pad on the surface of the PCB. Thus, the ground leads or tab of the electronic device package can be used as "thermal leads" to transfer heat from the device package to the thermal plane of the PCB. It should be noted, however, that the heat transferred to the thermal plane of the PCB must eventually be dissipated to the surrounding environment. If the thermal plane and thermal pads are incapable of adequately dissipating the heat to the surrounding atmosphere, a heat sink or other heat dissipating device may still be required. If required, a heat sink can be soldered to a thermal pad in direct, or indirect, thermal communication with the thermal plane. Although the thermal pads and heat sinks may ultimately provide adequate dissipation of the heat generated, these alternatives often consume valuable board space thereby increasing the size of the PCB or limiting the available board space for populating the PCB, both of which are undesirable side effects.
As noted, surface mount substrates or PCBs are desirable because of the efficient manufacturing process which can be used to assemble and populate such substrates with surface mount device packages. In a simplified form, the typical surface mount manufacturing process involves the following steps or operations. First, a solder paste is applied to the substrate or PCB using a mask to ensure that the paste is only applied to certain predetermined locations on the substrate and in certain predetermined amounts at those locations. After the solder paste has been applied, each electronic device package is placed on the substrate at a predetermined location in the solder paste. The location of the device package is critical because the device package is located such that its electrical leads will contact the exposed pads or leads on the substrate. Given the relatively small size of the leads on today's electronic device packages, even a small error in locating a device package could easily result in an electrical short or other misconnection. Although the solder paste is "tacky" (i.e., it has some limited adhesive quality) and will hold the device packages to some extent, the device packages are not secured to the substrate at this stage of the assembly process. Since the device packages are not yet secure, care must be taken not to dislodge or move the device packages while placing other device packages on the substrate or otherwise handling the substrate. Accordingly, heat sinks which are to be placed in direct physical contact with heat generating device packages typically can not be placed on the substrate at this stage of the process. Instead, these heat sinks typically must be placed on the substrate after the device packages have been secured to the substrate. Since the electronic device packages, however, are generally placed in physical locations on the board remote from one another, the device packages can typically all be placed on the board before any of the devices are secured to the substrate. Once all the device packages have been placed on the substrate, the entire assembly is heated in order to secure all of the device packages at once. This heating operation heats the solder in the solder paste causing the solder to melt and flow. After a predetermined time for the heating operation, the assembly is then allowed to cool and the solder reforms or hardens thereby securing the electronic device packages to the substrate. Once the device packages are secured to the substrate, other devices such as heat sinks can be placed on the PCB without any danger of dislocating the device packages from their correct, and now set, positions. These devices can then be secured by additional heating operations.
It is an object of the present invention to provide a method and apparatus which will allow a heat sink, which will ultimately be in direct thermal contact with a surface mount heat generating electronic device package, to be placed on the substrate in the surface mount assembly process before the device packages are secured to the substrate. Such an inventive method and apparatus will thereby allow for the device packages and any required heat sinks to be placed and secured to the surface mount PCB in a unified operation instead of in numerous distinct sub-operations.